The Bumpy Road to Net Zero Energy

It’s easy to lose the longer perspective when we are involved in day-to-day efforts to save energy. That’s why I want to focus on a single home—Danny Parker’s house in Florida—to illustrate the bumpy road to zero energy use. The figure on the right shows 21 years of electricity use, along with many of the major events in Danny’s household. (Follow the green line.)

When the Parkers moved into the house in 1989, Danny and his wife used roughly 10,000 kWh per year. (By coincidence, that’s also the national average residential electricity use.) Over the next 20 years, Danny installed many conservation measures and improvements. He added insulation, sealed ducts, installed a whole-house fan, replaced the refrigerator and air conditioner, and installed a PV-powered pump for the swimming pool. He also replaced all the incandescent lights with CFLs.

There were other events affecting energy use. The babies arrived— first Sarah, then Wade. In 1998, the Parkers added 500 square feet of floor area. In 2005, the children—no longer babies—convinced their parents to buy a digital video recorder (DVR) and a flat-screen TV. In 2006, Danny bought an energy feedback device, though it wasn’t clear if anybody besides him understood it. Finally, in 2009, the Parkers installed a 5kW PV system. Meanwhile, appliances were being replaced again. In 2010, the Parkers replaced the “new” refrigerator. The “new” air conditioner was replaced in 2010.

Over those 21 years, the Parkers’ energy use fell about 50% through efficiency improvements. They then eliminated the remaining 50% of grid-supplied power by installing a PV array. Now, in 2011, the Parkers’ house is exporting electricity.

The Parkers’ house was in no way special, yet it was able to reach net zero electricity use through a combination of familiar efficiency measures and investments in renewable energy. Arguably, many millions of households around the country could achieve similar results.

This rare perspective of 21 years of energy use reminds us that we shouldn’t treat a house as a static object. We can see that the progress of the Parker household toward net zero electricity consumption is hardly smooth, and many of the bumps don’t seem to correspond to particular technological events. Some of the bumps are good: The birth of two kids seemed to raise electricity use. That’s not really a surprise, but we shouldn’t forget this connection. The physical size of homes grows, too; this house increased roughly 25%. Nevertheless, we don’t see much increase in electricity use. Perhaps that’s because Danny was careful in the design and construction of the addition. Major appliances, such as the refrigerator and air conditioner, were replaced twice over the two decades. This is a reminder that efficiency experts should expect to visit each home several times in the path to net zero energy.

Of course one can argue that the Parkers’ home is not a fair example for comparison to an average home, and that we ignored natural-gas energy use. These are valid objections, but I don’t think they detract from the basic conclusion that a combination of vigorous conservation measures and appropriate use of renewable sources can achieve some—if not all—of our climate mitigation goals.

This is great data! I do have a question about the source energy graph.

It looks like you have been able to bring the monthly source energy use down to nearly 1 MBtu/month. Given that the electricity use has become a net exporter, and the only apparent gas use left is for water heating, does that mean you are using about 12-14 MBtu/yr (120-140 Therms/yr) for water heating?

That seems to be in contrast with what the sub-meter was showing. You mentioned the sub-meter showed only 18 Therms/yr for the water heating.

No constant thermostat setting was used. Indeed, an effort was made to raise and lower the thermostat to attempt to minimize loads. Air conditioning was only used from May - September and in some years from July - August.

Similarly, although the building was heated to 70 -72 F often, gas was used in most years. Only in 2010 to 2011 was all electric heat pump heating done with the home's single mini-split system. The mini-split heat pump now uses only about as much electric power as the furnace's electric blower!

What a wonderful set of data. Is it safe to assume that thermostat settings were constant across this period or did the homeowners make an effort to reduce their consumption by raising (or lowering) the homes temperature? Since Jan. is the coldest month in this location at 72/50F there should be an AC load which I would expect to be tough to support with PV, even in a location with this much sunshine.

I always assume that zero energy includes all fuels. There is the case of zero electricity, but that is a lesser goal, but not necessarily an unworthy one. Here is my home's progress with total energy, both gas and electric included. 90% of the way there to get to actual zero:

On the water heating system: Yes, I use a 160 F cutoff with a mixing valve set to 120 F after the tankless gas water heater. That does improve solar fraction since machine related draws (washing machine, dishwasher) are drawing 120 F rather than 160 F. As detailed in other work we've done, some of these draws are not strictly heated by the water heater anyway as they are drawing the water between the water heater and the end use that can be substantial depending on the length and diameter of the pipe. Secondly, draws looking to bring hot water to a distant end use (eg. bedroom sink) will be drawing 120 F water rather than 160 F water to simply get hot water there to wash your face.

The key thing in my system is with a tankless gas backup, it should be elevated above the solar storage tank so that the hot water naturally migrates via buoyancy to the tankless water heater inlet so that it does not come on, even during the early part of the hot water draw.

Thanks. I am just a practitioner. I don't do any logging after we do work. it does seem to me that if we aspire to net zero energy, we need to be stubborn and say net zero for all non renewable fuels though. if the sun or wind were considered as energy sources, we woudl never reach net zero, but an all electric house will always be net zero if we only look at gas consumption. ;-)

What high limit shut off temperature did you use for your solar storage tank? I have a solar water heater too, and recently met a guy who suggested I set the high limit for solar storage at 160, then install a mixing valve between the solar tank and the house so water delivered to faucets came at 120 degrees. He said that would maximize my solar fraction. What do y'all think of that idea?